Avoid setting transitions in-place for cached maps when observed

R=verwaest@chromium.org
BUG=

Review URL: https://codereview.chromium.org/246523004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20900 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/objects.cc

@@ -5816,7 +5816,7 @@ void JSObject::SetObserved(Handle<JSObject> object) {
   if (transition_index != TransitionArray::kNotFound) {
     new_map = handle(old_map->GetTransition(transition_index), isolate);
     ASSERT(new_map->is_observed());
-  } else if (old_map->CanHaveMoreTransitions()) {
+  } else if (object->HasFastProperties() && old_map->CanHaveMoreTransitions()) {
     new_map = Map::CopyForObserved(old_map);
   } else {
     new_map = Map::Copy(old_map);

test/mjsunit/regress/regress-observe-map-cache.js

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax --enable-slow-asserts
+
+function f() {
+  var x = new Array(0);
+  x[-1] = -1;
+  Object.observe(x, function() { });
+}
+
+f();
+f();